This invention relates generally to lithography.
In extreme ultraviolet lithography, a multilayer mask blank exhibits localized reflectivity to define features which may be transferred from the mask to a semiconductor wafer in repeatable fashion. The use of extreme ultraviolet lithography enables relatively smaller feature sizes to be transferred.
Mask aerial images may be subject to the so-called proximity effect associated with the limitations of the exposure projection lens. The proximity effect is an optical effect that causes features at different pitches to be printed at different critical dimensions for the same exposure dose. The distortion of the mask image may also include line end shorting, due to the resolution limit of the projection lens.
Proximity correction and mask pattern distortion correction may be accomplished by modifying the mask design. For example, in order to adjust the pitch dependent feature's critical dimension, lines on the mask may be designed either larger or smaller than the size desired to be transferred, to compensate for the proximity effect. In the case of line end shorting, additional chromium absorber may be added to that end. These modifications, in many cases, push the mask fabrication to its limit. The problems include not only a huge amount of data handling, but also the resolution limitation in mask patterning due to small correction features, very long inspection time, and possible data confusion.
Thus, there is a need for a better way to correct the proximity effect in extreme ultraviolet lithography masks.